Particularly owing to the very small size and the large number of microcomponents present on a given layer, each transferred layer, that is to say each wafer comprising the layer, must be positioned on the final substrate (the first wafer on its own or already comprising other transferred layers) with great precision in order to comply with very strict alignment with the underlying layer. It may furthermore be necessary to carry out treatments on the layer after its transfer, for example in order to form other microcomponents, in order to uncover microcomponents on the surface, in order to produce interconnects, etc., these treatment operations also having to be carried out with very great precision in relation to the components present in the layer.
The transfer of a layer onto the final substrate involves molecular adhesion bonding between a first wafer and a second wafer, of the type described above, the second wafer generally being subsequently thinned. During the bonding, the wafers are mechanically aligned. Three types of alignment defects may be observed between the two wafers, namely alignment defects of the “offset” or “shift” type, of the rotation type and of the radial type (also known by the name “run-out”, magnification error or deformation error).
When a sequence of lithography steps is carried out on a single wafer, these types of defects are generally corrected by using a compensation algorithm in a lithography machine in order to preserve perfect alignment between each step.
During alignment between two wafers with a view to bonding, the alignment defects of the shift and rotation types can be compensated for mechanically by modifying the relative position of the wafers with respect to one another in the bonding machine. However, the alignment defects of the radial type cannot be compensated for by such repositioning of the wafers.
The radial misalignment occurs when two wafers to be aligned have different radial expansions; the radial expansions may be due to the fact that each wafer has undergone a different process of fabricating microcomponents and/or the fact that the processes applied to one or other of the wafers can lead to them being strained and make their dimensions vary on the microscopic scale, as in the case for example of layer deposition or oxidation which induces tensile strains for the wafer.
FIG. 1A illustrates the alignment between a first wafer 10 and a second wafer 20 with a view to bonding them by molecular adhesion. A first series of microcomponents 11 has been formed beforehand on the bonding face of the first wafer 10, while a second series of microcomponents 21 has been formed beforehand on the upper face of the second wafer 20, intended to be bonded to the first wafer. The microcomponents 11 are intended to be aligned with the microcomponents 21 after bonding of the wafers.
In the example described here, however, the first and second wafers have different radial expansions, thus creating a radial misalignment between these wafers which, after bonding, results in offsets between the majority of the microcomponents such as the offsets Δ11, Δ22, Δ33, Δ55, Δ66 or Δ77 indicated in FIG. 1B (respectively corresponding to the offsets observed between the pairs of microcomponents 111/121, 112/122, 113/123, 115/125, 116/126 and 117/127).
The radial expansions responsible for the radial misalignment between two wafers are generally homogeneous over the wafers, thus creating a radial misalignment which evolves (i.e. increases) quasi-linearly between the centre and the periphery of the wafer.
The radial misalignment may be corrected in particular during the conventional steps of forming components by photolithography, by means of a correction algorithm and as a function of misalignment measurements carried out on a wafer.
The correction of the radial misalignment, however, can be carried out only on one wafer on its own. Moreover, when the production of the microcomponents involves a step of bonding between two wafers as is the case when producing three-dimensional structures, it is no longer possible to carry out corrections relating to the radial misalignment.
Furthermore, when a layer of microcomponents is transferred onto a final substrate having a first layer of microcomponents, it is very important to be able to minimize the radial misalignment between these microcomponents of each of the layers when they are intended to be interconnected together. It is not in fact possible to compensate by lithography for the misalignments existing between the microcomponents of the two layers in this case. Thus, the present invention now provides a solution to these problems.